We seek 5-year support to extend the collaborative Puerto Rican Infant Twin Study ("PRINTS") for characterizing the interplay of genes and the family environment in the development of behavioral problems and psychopathology in preschool children. We have completed the first wave of assessment on a population-based sample of 898 Puerto Rican twin pairs born in 2000 and 2001. Twins are ascertained in the first 6 months of life. We have interviewed more than 80% of their mothers. A second wave of assessment is currently in progress. The proposed extension will allow us to ascertain a third cohort and to follow up the existing cohorts through their preschool years. Early assessment focuses on temperament and mother-child interaction. In-home pre-school assessment will focus on problem behaviors that, in older children, comprise the principal common diagnostic categories of child mood and behavioral disorder. The data will provide estimates of the early prevalence of psychiatric disorder in a population-based sample of pre-school children and permit us to characterize the developmental pathways from early inherited differences in temperament to later problem behavior. We hypothesize that there are 3 pathways through which genes influence later behavioral outcomes: 1) a main (pleitropic) effect through which the genes that influence early temperament also affect risk to behavioral disorder through common neurogenetic pathways; 2) evocative genotype environment correlation through which parental reaction to infants' temperament mediates the relationship between early temperament and later problem behavior; 3) genotype x environment interaction through which early genetic differences in temperament reflect differential sensitivity to aspects of the family environment that produce later problems in vulnerable children. DNA samples will be obtained to aid zygosity diagnosis and for subsequent genoryping for markers that have been shown to affect sensitivity to environmental adversity.